1. Field of the Invention
The present invention relates to a liquid crystal display (LCD), and more particularly, to a liquid crystal display having an electrode used as an alignment layer, and a method of fabricating the same.
2. Description of the Related Art
In general, a cathode ray tube (CRT) has been most widely used among display devices for displaying image information on a screen until now, but has many inconveniences owing to a large volume and weight compared with the display area.
With the development of electronic industries, the display device whose usage was limited to a TV Braun tube and so forth, is has been expanded for use in a personal computer, a notebook, a wireless terminal, a vehicle instrument panel, an electronic display board and the like. Also, due to the development of information communication technology, since it is possible to transmit a large amount of image information, the importance on a next generation display device capable of processing and displaying the large amount of image information has constantly been increasing.
Many requirements exist for such a next generation display device; the next generation display device must be lighter, thinner, shorter and smaller, have a high luminance, a large-sized screen, low power consumption and a low price. Among such next generation display devices, the LCD has recently gained in popularity.
The LCD has excellent resolution, better than other flat displays, and a fast response speed compared to that of the CRT in implementing a moving picture.
Twisted nematic (TN) mode LCDs have become extremely popular and widely used among LCDs at the present time.
In the TN mode LCD, after alignment layers are respectively formed on two substrates, directors of liquid crystal molecules adjacent to the lower substrate are perpendicular to directors of liquid crystal molecules adjacent to the upper substrate.
When an electric field is not applied, liquid crystal molecules of the TN mode LCD have a structure in which longer axes of the liquid crystal molecules are twisted in a spiral fashion. The liquid crystal molecules have a pitch and are parallel to the substrate.
Recently, LCDs employing a new mode are being actively researched. As examples of the new mode, there are in-plane switching (IPS) mode, optically compensated birefringence mode, etc.
The IPS mode LCD generates a horizontal electric field so as to drive the liquid crystal molecules in a horizontal state with respect to the substrates by forming two electrodes on an identical substrate and applying a voltage between the two electrodes. In other words, the longer axis of the liquid crystal molecule does not stand up with respect to the substrates.
A related art in-plane switching (IPS) mode LCD will now be described in detail with reference to the accompanying drawings.
FIG. 1 is a sectional view of a related art LCD.
Referring to FIG. 1, a gate electrode 121 of conductive material such as metal is formed on a first transparent substrate 111. A gate insulator 130 such as silicon nitride (SiNx) or silicon oxide (SiOx) is formed on the gate electrode 121 and the first transparent substrate 111.
An active layer 141 of amorphous silicon is formed on the gate insulator 130 disposed on the gate electrode 121. Ohmic contact layers 151 and 152 of impurity-doped amorphous silicon are formed on the active layer 141.
Source and drain electrodes 161 and 162 of conductive material such as metal are formed on the ohmic contact layers 151 and 152. The source and drain electrodes 161 and 162 form a thin film transistor (TFT) together with the gate electrode 121.
A passivation layer 170 of silicon nitride (SiNx), silicon oxide (SiOx) or organic insulator is formed on a resultant structure including the source and drain electrodes 161 and 162. The passivation layer 170 has a contact hole 171 exposing the drain electrode 162.
A pixel electrode 181 of transparent conductive material is formed on a pixel region of the passivation layer 170 and is electrically connected to the drain electrode 162 through the contact hole 171.
A first alignment layer 191 of polyimide whose surface is aligned in a predetermined direction is formed on the pixel electrode 181.
At this time, the gate electrode 121 is connected with a gate line, and the source electrode 161 is connected with a data line. The pair of gate lines and the pair of data lines perpendicularly crossing the gate lines define a unit pixel region.
Meanwhile, an upper substrate including a second transparent substrate 110 is arranged over and spaced apart by a predetermined distance from the lower substrate constructed as above and including the first transparent substrate 111.
A black matrix layer 120 for preventing light leakage in portions other than the pixel regions is formed at a portion corresponding to the TFT beneath the second substrate 110.
A color filter layer 131 is formed beneath the black matrix layer 120 and the second substrate 110. The color filter layer 131 includes red (R), green (G) and blue (B) color filters repeatedly arranged, each color filter corresponding to one pixel region.
A common electrode 140 of transparent conductive material is formed beneath the color filter layer 131. A second alignment layer 150 of polyimide whose surface is aligned in a predetermined direction is formed beneath the common electrode 140.
A liquid crystal layer 190 is interposed between the first alignment layer 191 and the second alignment layer 150. Liquid crystal molecules of the liquid crystal layer 190 have an initial alignment direction, which is decided by the alignment direction of the first and second alignment layers 191 and 150.
The alignment layer forming process for determining the initial alignment direction of the liquid crystal molecules will be described in more detail hereinafter.
First, the alignment layer forming process includes the steps of coating a polymer film used as a raw material of the alignment layer and aligning the coated polymer film in a predetermined direction.
The alignment layer mainly uses a polyimide-based organic material, and is aligned by a rubbing method.
The rubbing method includes the steps of: coating a polyimide-based organic material film on a substrate; vaporizing a solvent contained in the coated polyimide-based organic material film at a temperature of 60-80° C.; hardening the polyimide-based organic material film at a temperature of 80-200° C. to form a polyimide alignment layer; and rubbing the polyimide alignment layer using a rubbing cloth such as a velvet in a predetermined direction to form an alignment direction.
Referring to FIG. 2, a method of fabricating the LCD of FIG. 1 will now be described in detail.
First, upper and lower substrates having the construction described in FIG. 1 are fabricated (S100).
Next, a cleaning step (S110) is performed to remove foreign substances on the upper and lower substrates each having various patterns formed thereon. After that, an alignment layer-printing step (S120) for printing polyimide (PI), which is a raw material of the alignment layer, on the upper and lower substrates is performed using an alignment layer-printing apparatus.
Afterwards, an alignment layer-plasticizing step (S130) in which a high temperature heat is applied to the printed polyimide to vaporize a solvent and harden the polyimide is performed.
Next, an alignment layer-rubbing step (S140) in which an upper surface of the plasticized alignment layer is rubbed in a predetermined direction by using a rubbing apparatus to form a groove is performed.
After the alignment layer-forming step is completed, a seal pattern serving as an adhesive is formed at an edge of the upper substrate except for a liquid crystal injection inlet, and a spacer is dispensed on the lower substrate (S150).
Next, the prepared two substrates are attached to each other with a spacing therebetween. In the attaching of the two substrates, a preciseness of a few micrometers is required to prevent light from being leaked when the preciseness is out of the given value (S160).
After that, a cell-cutting step (S170) for cutting the opposing substrates attached into a plurality of unit cells is performed. The cell-cutting step (S170) is performed to cut the completely attached substrates to a necessary size, and includes a scribing step for forming a cutting line on the outer surfaces of the upper and lower substrates, and a breaking step for dividing the attached substrates into the unit cells by applying an impact on the scribed line.
Finally, a liquid crystal is injected into a space between the two substrates cut in a unit cell, and a liquid crystal injection inlet is sealed to prevent the injected liquid crystal from being leaked, thereby completing an LCD (S180).
In the above LCD, the liquid crystal has a physical characteristic, which is varied with the alignment state of the liquid crystal molecules. The physical characteristic of the liquid crystal causes a difference in the response by an external force such as an electric field.
Owing to the aforementioned property of the liquid crystal molecules, it is an essential technique to control the alignment of the liquid crystal molecules for research on the physical property of the liquid crystal molecules and constitution of the LCD.
Especially, the rubbing process for allowing the liquid crystal molecules to be aligned uniformly is an important factor to determine the normal operation of the LCD and the uniform display characteristic of the screen, and the rubbing process accordingly has been the subject of much research.
The above-described rubbing method has advantages in that since the orientation treatment is easy, it is suitable for mass production and stable orientation can be made.
However, the usage of a roller having a defective rubbing cloth in the above rubbing process causes a rubbing failure.
In other words, since the rubbing method using such a rubbing cloth is performed by direct contact between the alignment layer and the rubbing cloth, various problems may occur. These problems may include contamination of liquid crystal cell due to the occurrence of particles, fracture of the thin film transistor (TFT) device, the need for an additional cleaning process after the rubbing process, non-uniformity of the alignment direction in a large-sized application, etc. All of these problems serve to lower the production yield of the LCD.
Also, since the above rubbing method includes coating the organic material film and rubbing the coated organic material film, an additional process is required, and cost of the alignment layer is required, thereby increasing the fabrication costs.